Time Domain Reflectometers (TDRs) have proven to be the preferred instruments for the measurement of permittivity in various media, including soils. This popularity arises from the independence of propagation time from the electrical conductivity of the medium. Volumetric water content measurements can be calculated with high confidence from permittivity, using techniques described by Anderson in U.S. Pat. No. 6,657,443, which makes these instruments highly suitable for measuring the moisture environment in the root zone of growing crops. Many other methods of soil water content measurement are plagued by errors attributed to the electrical conductivity of the soil.
A further advantage of TDRs is that they can be inserted into the soil without excavation and thus can gather transpiration data without severely disturbing the delicate transpiration system comprised of the small roots and soil structures around them. This is often a critical requirement for horticultural research and for monitoring the root zone of food crops.
The use of Time Domain Reflectometers has been limited by their high cost. A further drawback has been the impractical deployment of TDR sets in agricultural and research fields due to their coaxial cables and power supply lines which must be strung through the crops. Both of these drawbacks have been resolved with the introduction of an integrated Time Domain Reflectometer wherein the step function generator and waveform digitizer are located immediately at the incident end of the waveguide, as disclosed in U.S. Pat. No. 6,831,468 by Anderson et al.
In that disclosure the waveguide is single-ended. The step function propagates along a single conductor in the presence of a reference ground plane or conductor. Although that 2-conductor waveguide provides a useful propagation path, it lacks balance and does not maximize the volume of medium subjected to the permittivity measurement. In practical systems, the reference ground plane is often divided into two rods parallel to the propagation rod and spaced equally on either side of it, thus forming a 3-rod waveguide. This improves the spatial balance of the propagating wave and facilitates the permeation of additional soil volume in the measured sample. Many traditional TDR probes are fashioned from three rods to achieve these advantages. The coaxial center conductor is wired to the center rod and the sheath is wired to the outer two rods. However, the resulting 3-rod probe is more difficult to insert into the root zone and still is not optimum for sampling volume.
In order to reduce the insertion force in a balanced waveguide and to improve its sampling volume some waveguide probes are built with baluns in them, a balun being a transformer used to convert between a balanced signal and a single-ended one. In other words, these devices convert the unbalanced single-ended coaxial cable into a balanced differential 2-wire transmission line. The waveguide takes the form of two rods with a balanced differential wave propagating on them. The sampled soil volume is improved by about 40% over a 3-rod waveguide having the same width, and the insertion force is reduced considerably.
A disadvantage exists with the 2-rod waveguide. Since the balun is reactive, being an inductive device, it acts as a high pass filter. It cannot propagate the steady state part of the step function. Many researchers depend upon the long-term response of the reflected wave to derive the electrical conductivity of the soil. When a balun is deployed as part of a measurement system, the long-term step response diminishes severely and can easily droop to zero within a few tens of microseconds. Thus, balanced waveguides equipped with baluns have not been deployed where electrical conductivity measurements require high confidence.
The ideal TDR waveguide system would employ all of the following:
1. A balanced differential wave with uniform electromagnetic (EM) field characteristics relative to the poles of the waveguide;
2. No more than two rods for easier insertion and higher sampling volume; and
3. No droop in the steady state step response, that is, no balun.